Dark Blue Tone vs Cerulean Blue Deep

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Cerulean Blue Deep is noticeably lighter, more saturated, cooler than Dark Blue Tone. They are different kinds of paint: Army Painter Dark Blue Tone is a wash/shade and Golden Cerulean Blue Deep is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.

Can I use Cerulean Blue Deep instead of Dark Blue Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Cerulean Blue Deep is noticeably lighter, more saturated, cooler than Dark Blue Tone. They are different kinds of paint.

Can I use Dark Blue Tone instead of Cerulean Blue Deep?

The same difference applies the other way round: Dark Blue Tone is noticeably darker, more muted, warmer.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.